Metal sulfide resistance elements



March 27, 1956 RESISTANCE. (LOG) Filed July 12, 1952 Fig.2.

[75C. TEMPERATURE Inventor: F'recleric: RQuinn,

by His Attorney.

United States Patent METAL SULFIDE RESISTANCE ELEMENTS Frederic R. Quinn, Red Hook, N. Y., assignor to General Electric Company, a corporation of New York Application July 12, 1952, Serial No. 298.495 7 Claims. (Cl. 201--63) The present invention relates to metal sulfide resistance elements for use in electric circuits. It is particularly concerned with electrical resistance elements comprising a combination of a plurality of metal sulfides.

In my copending application Serial No. 106,293, filed July 22, 1949, now Patent No. 2,609,470, and assigned to the same assignee as the present invention, there are dcscribed temperature-responsive resistance elements essentially composed of a sintered mixture of metal sulfides including molybdenum sulfide, a sulfide of a metal of group 2 of the periodic table and a major portion of a third metal sulfide which is referred to therein as a base sulfide and which determines the specific resistivity characteristics of the element. it was found that the presence of the molybdenum and group 2 metal sulfides was essential in order to obtain products characterized by a high degree of chemical stability and reproducible, stable, resistivity characteristics.

The present invention is based on the discovery that further improvements can be realized by employing as the group 2 metal sulfide component of the sintered sulfide compositions, both zinc sulfide and at least one sulfide selected from the group consisting of barium sufide and calcium sulfide.

More particularly, it has been found that the sulfide elements of the present invention will retain their stable resistivity characteristics when operated at high voltages and at more elevated temperatures than are generally possible with those described in my copending application. Additional advantages and features of the present inven-. tion will become apparent from the following description taken in connection with the accompanying drawing wherein Fig, 1 is a cross-sectional view of one form of resistance element embodying the present invention and Fig. 2 illustrates the temperature-resistance characteristics of a representative element.

The elements of the present invention may be provided in any of a number of forms. A particularly useful element is that shown in Fig. 1 of the drawing in which one of the metal leads or contacts for electrically connectmg the. element into an electrical circuit is in the form of a case or cup 1 and the second lead 2 is in the form of a wire centrally positioned within the cup 1. The sulfide composition 3 is pressed into the case around the center lead 2 and sintered in situ. The upper end of the case, is sealed with a suitable electrically insulating e ing m eri l 4.

While the advantages of the present invention will be described with specific reference to a resistance, element containing silver sulfide as the base sulfide, i. e., as the major ingredient, it will be understood that any desired base metal sulfides can be substituted fOr the silver sulfide for the purpose of obtaining resistance materials or elements having different temperature-resistance character istics. Examples of suitable base sulfides include group 2 metal sulfides other than the. zinc, barium or calcium sulfides O a ulfide of sodium, potassium, copper, platinum, mercury, lead, antimony, aluminum, chromium,

nickel, etc. All of these sulfides exhibit a sharp break in electrical resistance at certain temperatures or over a narrow range of temperatures, and when they are present in major proportions in the metallic sulfide mixtures, they impart to these mixtures the same temperature-resistance characteristics. The temperature-resistance curve for silver sulfide, for example, which is plotted in Fig. 2 of the drawing shows a sharp break at C.

The method of preparing the materials and elements of the present invention is substantially the same as that set forth in my copending application. All of the metal sulfides are preferably pulverized to a finely divided state and mixed in the prescribed proportions. The mixtures are then pressed to the desired form and shape employing a pressure sufiicient to obtain a product of a density such that its resistance characteristics are unaffected by further pressure. Ordinarily a pressure of at least about 20,000 pounds per square inch is sufiicient. During the pressing operation, electrical connections in the form of leads are pressed into contact with the mixed sulfide body and thereafter the compacted body is sintered at an elevated temperature above the break point temperature of the mixture. It is also essential that the sintering temperature be above, any temperature which will normall) be encountered during use of the resistance elemen s.

After sintering, the elements are usually subjected to a stabilization process which comprises alternately passing a direct current through the element first in one direction and then in the opposite direction until no further change in the resistance of the element is noted. During this process any free sulfur present in the sintered mixture is deposited out on the leads in the form of stable sulfides. In general, the current flow in each direction is so regulated that the stabilized product will have the same resistance value regardless of the direction of current'fiow. The migration of sulfur to the vicinity of the leads or contacts during the. stabilization also bonds the leads or contacts mechanically and electrically to the sintered sulfide mixture. To assure this latter result, a smail amount of free sulfur is normally included in the sulfide mixture employed in making the elements. The added sulfur may range from about 0.03% to as high as 10% but usually will not exceed about 0.5% by weight based on the total weight of the metal sulfides.

The metal leads employed in the practice of the present invention should be characterized by a useful life equal to that'of the sulfide body per se. The leads or contacts should posses the property of reacting only to a limited extent with free sulfur to form on the surface of the lead a thin, stable, protective film of metal sulfide which apparently prevents further reaction between the metal and the sulfur or sulfide content of the sulfide bod. example of one of the least reactive lead metals is piatinuin and this metal can readily be used for the manufacture of electrical elements for alternating-current applications. However, it should not ordinarily be used for accurate direct-current applications due to the fact that when so used a continuous polarization of one of the platinum leads can take place over a periodof time with a gradual increase in the over-all electrical resistance of the element.

Examples of suitable, leads or contacts of the type which are sulfided readily but only to a limited extent are those of molybdenum, chromium, nickel and nickel or chromium alloys. While such metals or alloys, or metal leads of steel or the like plated or coated with such metals or alloys, are at first attacked by the sulfur, the attack is not continuing one as in the case of copper and the electrical h ra t r t of h le ent come c nst upon th formation of a stable, substantially indestructible. and protective film of metal sulfide on the surface of the leads or contacts, which sulfide layer also exhibits a constant and stable electrical resistance characteristic. Leads of or surfaced with molybdenum, nickel and a nickel alloy, e. g., Nichrome (80% Ni, Cr) or Chrornaloy Ni, 15% Cr, bal. Fe), and to a lesser extent aluminum leads are preferred. All of these have the desired characteristics with regard to the freedom of complete sulfidation and disintegration. Tantalum leads are subject only to the disadvantage that it is quite difiicult to form a sulfide layer suificiently thick to act as a mechanical bond between the lead and the sintered sulfide mixture. This is also true of tungsten and thorium-coated tungsten leads. While elements in which one or both of the leads are of this type are preferably first stabilized by the polarization process, once they are so stabilized, their electrical characteristics for D.-C. applications remain constant for an indefinite period of time and are not affected by subjection to subzero temperatures or to elevated temperatures below the sintering temperatures. Any of these leads may also be used for elements intended for A.-C. applications and in such cases the polarization process may be omitted or used only to the extent necessary to obtain a good bonding of the leads to the sintered sulfide.

Following the above-described procedure, the products of the present invention are prepared from sulfide mixtures including, by weight, from 2 to 20% molybdenum sulfide, from 2 to 20% zinc sulfide, from 2 to 15% of at least one sulfide selected from the group consisting of barium sulfide and calcium sulfide, and a major proportion, preferably from 60 to 80%, of an additional metal sulfide exhibiting a definite break in its temperatures-resistance curve.

Preferably, the calcium sulfide content of the mixtures is below 10%, by Weight, and the zinc sulfide content from 5 to 15%, by weight. Also, for best results, the molybdenum sulfide content should not be more than about twice the zinc sulfide content of the mixtures.

It has been found that by employing the above-enumerated sulfides in the proportions indicated, resistance elements are obtained possessing characteristics not possessed by elements containing only a single essential sulfide as a stabilizing ingredient. In describing the advantages of the present sulfide mixtures and elements in comparison with those described in my copending application, it is to be understood that all of the elements being compared contained, in addition to the essential sulfide or sulfides, a base sulfide, such as silver sulfide, and quantities of molybdenum sulfide within the proportions hereinbefore indicated.

One of the advantages of the elements of the present invention is the fact that they can be operated at higher voltages. For example, elements containing both zinc and calcium sulfide or both zinc and barium sulfide or all three of these sulfides can be operated at voltages up to 125 volts whereas elements containing only one of these essential sulfides are usually limited to operating voltages not exceeding about 60 volts. The zinc sulfide-calcium sulfide elements will withstand operating temperatures from 100 to 150 C. higher than the corresponding elements containing only zinc sulfide or only calcium sulfide. The zinc sulfide-barium sulfide elements, on the other hand, will withstand operating temperatures at least 200 C. higher than the elements containing only one of these sulfides. The operating or maximum operating temperatures for elements containing all three of the sulfides, zinc sulfide, barium sulfide, and calcium sulfide, are ordinarily intermediate the maximum operating temperatures for the zinc-barium sulfide and zinc-calcium sulfide mixtures.

In combination with the remaining sulfides and in the proportions set forth hereinbefore, it has been found that an increase in the amount of zinc sulfide is accompanied by a proportional increase in the low or room temperature resistance values of the sulfide mixtures. In addition, the sintering temperatures as well as the maximum operating temperatures of the mixtures increase with increased zinc sulfide content. It has also been found that during manufacture of the elements, those containing both zinc and calcium sulfides are less sensitive to high humidity conditions than are elements containing only calcium sulfide as the stabilizing or essential sulfide.

The best bonding characteristics, insofar as the bond between the sintered body of sulfide resistance material and the leads are concerned, are normally obtained when there is at least a small amount of calcium sulfide present in the mixture. This is particularly true when the leads are nickel or nickel-chromium alloys. In addition, those elements containing calcium sulfide appear to exhibit somewhat better D.-C. operating characteristics than the elements containing only zinc sulfide or only zinc and barium sulfides. As has been previously indicated, either calcium sulfide, barium sulfide, or both, must be present along with the zinc sulfide in order to obtain the best high temperature operating characteristics.

In order that those skilled in the art better may understand how the present invention may be carried into effect, there is set forth hereinafter, by Way of illustration but not by way of limitation, a detailed description of specific polysulfide elements within the scope of the present invention.

Example 1 A resistance element particularly useful for direct current applications was made from a mixture consisting, by weight, of 12 parts finely divided silver sulfide, 1.2 parts calcium sulfide, 0.8 part zinc sulfide, 0.5 part molybdenum disulfide and 0.5 part free sulfur. Portions of this mixture were placed in Nichrome cups and a molybdenum wire lead centrally positioned in each cup in such a man ner that it was separated from the sides and bottom of the cup by the mixture. The sulfide mixtures were then consolidated by a pressure of at least 20,000 p. s. i. and the pressed elements in which the Nichrome cup would function as one lead and the molybdenum wire as the other were sintered at a temperature of 500 C. This temperature is approximately higher than the optimum sintering temperature for a similar composition containing no zinc sulfide.

The sintered elements were subjected to the stabilization process described in my above-mentioned copending application Serial No. 106,293, now Patent 2,609,470, by passing a direct current through the element between the Nichrome case and the molybdenum wire and periodically reversing the direction of current flow until the element exhibited the same resistance in both directions. The stabilization treatment was carried out at a temperature above C. which is the temperature at which silver sulfide elements exhibit a sharp break in their temperature-resistance curves.

The elements containing both zinc and calcium sulfide have less tendency toward exhibiting unsymmetrical resistance values than the elements containing only calcium sulfide and no zinc sulfide. For example, the element of the present example could be stabilized merely by passing a direct current through the element for one hour in each direction rather than continuously reversing the current for a period up to eight hours as required for elements containing only calcium sulfide as the essential sulfide. One reason for this result appears to be that when both the zinc and calcium sulfides are employed, it is possible to add more molybdenum sulfide without detrimentally lowering the high resistance of the element at normal or room temperatures. In elements containing only calcium sulfide as the essential sulfide, about 1% molybdenum sulfide is preferred in order to keep from lowering the room temperature resistance of the element below about 100,000 ohms. In the present elements, as much as 3% molybdenum sulfide can be included and the resultant element will still have a top room temperature resistance of about 250,000 ohms. The presence of this additional molybdenum sulfide appears to render the elements more susceptible to the stabilization treatment.

Elements containing only zinc sulfide as the essential sulfide have rather poor D.-C. operating characteristics with currents much above milliamps, with a tendency toward erratic resistance changes during the time the temperature of the element is carried through the break point. Since a current of at least about 20 milliamps is necessary to operate an inexpensive relay directly rather than through an amplifying system, the zinc sulfide-calcium sulfide elements, which are capable of continuously handling currents of milliamps or more without change in their physical characteristics, are particularly useful for direct relay operation.

Furthermore, the elements containing both zinc sulfide and calcium sulfide can be operated at higher voltages than corresponding elements containing only zinc sulfide or only calcium sulfide. For example, elements containing both of these sulfides in addition to silver sulfide and molybdenum sulfide operate satisfactorily at voltages up to 125 volts, whereas similar elements containing only one of these two sulfides are somewhat erratic in their operating characteristics at voltages much in excess of about volts.

A particular advantage of the elements containing zinc sulfide in addition to the calcium sulfide is that they are less susceptible to humidity conditions during their manufacture. In preparing elements from finely divided mixtures of silver sulfide, calcium sulfide and molybdenum disulfide and sulfur with no added zinc sulfide, satisfactory results with minimum rejects can ordinarily be obtained only when the relative humidity existing during their manufacture is 30% or less. On the other hand, by the inclusion of some zinc sulfide, acceptable elements can be made when the relative humidity is as high as Elements containing both zinc and calcium sulfide or zinc, calcium and barium sulfide also exhibit better bonding characteristics, particularly as regards leads in the form of a Nichrome shell or cup, than do the elements free of any calcium sulfides.

A number of the stabilized elements containing both zinc and calcium sulfides were placed on life tests which comprised alternately and repeatedly subjecting the elements to a temperature above the break point temperature and then to room temperature while continuously passing a direct current through the elements. No change in the resistance values of the elements was noted after 100,000 cycles. A number of the same elements were also heated to 500 C. for three months with no observable change in either their high or low temperature resistance values.

While the zinc-calcium sulfide element has been described with particular reference to silver sulfide as the base sulfide ingredient, it will be understood it is not limited thereto. Elements having diiferent break point temperatures can be obtained by substituting other base metal sulfides, such as the sulfides of cadmium, aluminum, nickel, chromium and platinum for the silver sulfide. This is also true of the zinc-barium sulfide elements or zinc-barium-calcium sulfide elements as the improved properties realized by the use of two or three group 2 metal sulfides are obtainable with any of the base metal sulfides.

Example 2 Elements containing mixtures of zinc and barium sulfides as the essential sulfide component are found to be particularly useful for alternating current applications. An element of this type was prepared by mixing 12 parts, by Weight, silver sulfide with 1.2 parts zinc sulfide, 0.8 part barium sulfide, 0.5 part molybdenum disulfide and 0.5 part free sulfur. A portion of the resultant mixture was placed in a Nichrome cup along with a carefully centered molybdenum Wire lead. The mixture was pressed, sintered and the sintered elements stabilized as described hereinbefore. The stabilization treatment in the absence of any calcium sulfide is somewhat more prolonged than in the case of the zinc sulfide-calcium sulfide elements requiring reversal of the current a greater number of times before all of the free or ionic sulfur is plated out on the leads.

Zinc sulfide-barium sulfide elements possess good operating characteristics at temperatures up to about 600 C. which is approximately 200 C. higher than the maximum operating temperatures of corresponding elements containing only barium sulfide or only zinc sulfide as the essential sulfide. The elements have the additional advantage over the barium sulfide elements in that the straight barium sulfide elements slowly and gradually expand or swell after operation for 6 months to a year at temperatures in the neightborhood of 400 C. The presence of the zinc sulfide in addition to barium sulfide apparently eliminates this physical instability.

The barium sulfide-zinc sulfide element possess excellent alternating current operating characteristics even when employing unbalanced leads, that is, one lead of Nichrome or a nickel or chromium alloy and the other of molybdenum. After proper stabilization, the resistance values of the elements are the same in both directions of current flow and the elements exhibit no significant changes in their electrical properties even after long periods of aging at elevated temperatures up to 500 C.

The elements containing only zinc sulfide and barium sulfide as the essential sulfides are best suited for A.-C. applications, due in part to the fact that when a direct current is passed through the elements for prolonged periods of time, there is a tendency for the bond between the Nichrome shell or cup and the sulfide body to break. This tendency can however be eliminated by substituting a small amount of calcium sulfide for some of the barium sulfide. As little as about 1 percent calcium sulfide is sufiicient for this purpose. It is desirable that the calcium sulfide content in all of the elements be kept low, preferably not over 10 percent, by weight, to avoid humidity sensitivity in the compositions and elements during their manufacture.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A stable electric resistance element comprising a body of resistance material comprising a compressed, sintered mixture of a plurality of metal sulfides including, by weight, from 2 to 20 percent molybdenum sulfide, 2 to 20 percent zinc sulfide, 2 to 15 percent of at least one sulfide selected from the group consisting of barium sulfide and calcium sulfide, and from 60 to percent of an additional metal sulfide exhibiting a definite break in its temperature resistance curve selected from the group consisting of sodium, potassium, copper, platinum, mercury, lead, antimony, aluminum, chromium, nickel and group 2 metal sulfides other than zinc, barium or calcium sulfides, and metal leads electrically connected to said resistance material.

2. A stable electric resistance element comprising a body of resistance material comprising a compressed sintered mixture of a plurality of metal sulfides including, by weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to 15 percent of at least one sulfide selected from the group consisting of barium sulfide and calcium sulfide, the calcium sulfide content of the mixture being not more than about 10 percent, and fro-m 60 to 80 percent of an additional metal sulfide exhibiting a definite break in its temperature resistance curve selected from the group consisting of sodium, potassium, copper, platinum, mercury, lead, antimony, aluminum, chromium, nickel and group 2 metal sulfides other than zinc, barium or calcium sulfides and metal leads electrically connected to said resistance material.

3. A stable electric resistance element comprising a body of resistance material comprising a compressed sintered mixture of a plurality of metal sulfides including, by Weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to 15 ercent barium sulfide, and from 60 to 80 percent of an additional metal '7 sulfide exhibiting a definite break in its temperature resistance curve selected from the group consisting of sodium, potassium, copper, antimony, aluminum, chromium, nickel and group 2 metal sulfides other than zinc, barium or calcium sulfides and metal leads electrically connected to said resistance material.

4. A stable electric resistance element comprising a body of resistance material consisting essentially of a compressed sintered mixture of a plurality of metal sulfides including, by weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to percent calcium sulfide, and from 60 to 80 percent of an additional metal sulfide exhibiting a definite break in its temperature resistance curve selected from the group consisting of sodium, potassium, copper, platinum, mercury, lead, antimony, aluminum, chromium, nickel, and group 2 metal sulfides other than zinc, barium or calcium sulfides and metal leads electrically connected to said resistance material.

5. A stable electric resistance element comprising a body of resistance material consisting essentially of a compressed sintered mixture of a plurality of metal sulfides including, by weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to percent of at least one sulfide selected from the group consisting of barium sulfide and calcium sulfide, the calcium sulfide content of the mixture being not more than about 10 percent, and from to percent silver sulfide and metal leads electrically connected to said resistance material.

6. A stable electric resistance element comprising a body of resistance material consisting essentially of a compressed sintered mixture of a plurality of metal sulfides including, by Weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to 10 percent calcium sulfide, and from 60 to 80 percent silver sulfide and metal leads electrically connected to said resistance material.

7. A stable electric resistance element comprising a body of resistance material consisting essentially of a compressed sintered mixture of a plurality of metal sulfides including, by Weight, from 2 to 20 percent molybdenum sulfide, from 2 to 20 percent zinc sulfide, from 2 to 15 percent barium sulfide, and from 60 to 80 percent silver 1 sulfide and metal leads electrically connected to said resistance material.

References Cited in the file of this patent UNITED STATES PATENTS 959,068 Phillips May 24, 1910 1,563,557 Coblentz Dec. 1, 1925 2,148,453 Fruth Feb. 28, 1939 2,609,470 Quinn Sept. 2, 1952 

1. A STABLE ELECTRIC RESISTANCE ELEMENT COMPRISING A BODY OF RESISTANCE MATERIAL COMPRISING A COMPRESSED, SINTERED MIXTURE OF A PLURALITY OF METAL SULFIDES INCLUDING, BY WEIGHT FROM 2 TO 20 PERCENT MOLYBDENUM SULFIDE, 2 TO 20 PERCENT ZINC SULFIDE, 2 TO 15 PERCENT OF AT LEAST ONE SULFIDE SELECTED FROM THE GROUP CONSISTING OF BARIUM SULFIDE AND CALCIUM SULFIDE, AND FROM 60 TO 80 PERCENT OF AN ADDITIONAL METAL SULFIDE EXHIBITING A DEFINITE BREAK IN ITS TEMPERATURE RESISTANCE CURVE SELECTED FROM THE GROUP CONSISTING OF SODIUM, POTASSIUM, COPPER, PLATINUM MERCURY, LEAD, ANTIMONY, ALUMINUM, CHROMIUM, NICKEL ADND GROUP 2 METAL SULFIDES OTHER THAN ZINC, BARIUM OR CALCIUM SULFIDES, AND METAL LEAD ELECTRICALLY CONNECTED TO SAID RESISTANCE MATERIAL. 